Concentration measurements for Pr, Nd and Dy using spectrophotometry

1. Pr, Nd and Dy single spectra.

Some of the rare earth elements (REEs) give a characteristic spectrophotometric signal. This is typically reported as the absorbance vs wave-length spectrum. Figures below show the different spectrum for Pr, Nd and Dy, the three REEs we are focusing on here. Though the absorbance range is relatively small, the spectra clearly presents characteristic peaks. In particular, for each REE a key peak is identified as $\lambda^*$.

Intensity of the peaks depend on the concentration, following the Beer-Lambert law. These figures correspond to solution of 1000 ppm of REE in acetate.

Independent peak for Dy

Independent peak for Pr

Independent peak for Nd

Each REE can be identified using its characteristic peak at $\lambda^*$, which has no contribution of the other REEs.

Note: There is a large fluctuation on the scale of the baseline.

2. Background subtraction

The height of the peaks are proportional to the REEs concentrations. The first thing to do is to subtract the background due to the reference solutions. For instance if the REE is in and aqueous solution a reference spectrum of water with no REE should be measure and subtracted from the spectrum of the REE in water. To minimize the influence of the solution, always try to measure a reference background spectrum.

Subtraction of a reference background

As an example, you can see here the spectrum of Nd in an aqueous solution, together with the reference spectrum for H2O (top figure). The new spectrum where the reference background has been subtracted is shown in the bottom figure. In this way the new spectrum to be analyzed is obtained.

3. Measuring the peak height

Since the baseline present large fluctuations, instead of measuring the peak's height to the baseline, the height is measured relative to the main surrounding minima.

Definitions of the height

The independent peak for Nd at $\lambda^ = 740$ nm is shown as an example, whose maximum value is indicated with a gray circle. In order to find the main minima a reference window is defined, centered at $\lambda^$: window = ($\lambda - \Delta$, $\lambda + \Delta$).

In this case the peak has a structure, with a smaller peak to the left and a shoulder to the right. These are ignored and the two main minima within the defined window are found, as indicated with black circles.

The height of the peak is then measured with respect to the straight line joining the two minima. The height is indicated as a dashed black line.

4. Calibration curves

Measuring the height of the peaks as a function of the concentration then provides a calibration curve for the different cases. The calibration curves for Pr, Nd and Dy in H2O and in acetate are shown below.

Sebastian Bustingorry, 01/2025.